It has become apparent in recent years that the incidence of lead in the atmosphere, the soil and other surrounding environmental areas occupied by human beings is greater than previously anticipated. It is now recognized that lead can be airborne and may contaminate areas in which workers are housed while performing their day-to-day employment tasks. It has also become recognized that as a result of the use prior to the 1978 of paint containing lead, substantial soil contamination around older school buildings and older houses has resulted. In addition, upon the demolition of older buildings which were painted with lead containing paint, particles of lead may become airborne thus creating a toxic environment for the demolition workers and other individuals within the general demolition area.
There are many industries and activities in which lead in one form or another is commonly used. For example, in shooting galleries where live ammunition is utilized lead projectiles are propelled from the weapons toward the targets. The propelling of the lead projectiles and their impact with the target and with the projectile restraining walls beyond the targets creates a situation in which particles of lead enter the ambient atmosphere and remain airborne for a substantial period of time.
It is also known that in many industries where radiation of one form or another occurs, lead has been used as a common material for shields to protect personnel from bombardment by nuclear radiation. The presence of such lead shields in any significant amount generates a situation where lead particles can contaminate the ambient atmosphere. A similar type of situation occurs in industries in which lead is an integral or component part of the activity or the resulting product. Typical examples of such instances are where soldering occurs such as in the manufacture of electronic devices, the mining of lead, the smelting of ores containing lead, the manufacture of batteries which contain lead therein, the refining of petroleum products and many other industries which those skilled in the art will recognize.
As a result of the extensive lead contamination of the ambient atmosphere, it becomes extremely desirable to monitor the lead content of the atmosphere to determine whether or not personnel present may be subjected to a sufficient amount of lead to constitute a toxic danger.
Typically monitoring the ambient under such circumstances is accomplished by passing the ambient air, or a portion thereof, through an appropriate filter having sufficiently small pores to capture the airborne lead particles or at least a substantial number thereof. The filter containing the lead particles can then be subjected to appropriate measurement to determine the lead content in the ambient atmosphere to determine whether or not sufficient lead particles exist to constitute a potential danger to personnel in that area. This measuring process can be accomplished on a periodic basis which is determined by the surrounding circumstances and the application involved. For example the measuring can be done on a daily basis with appropriate records kept to track changes in the airborne lead content of the ambient atmosphere on a day-to-day basis. Under certain circumstances, the activity involved may be such that the measuring should be conducted on an hour-by-hour type basis or even more frequently in order to detect the existence of an immediate problem which may have occurred as a result of a malfunction of manufacturing processes or equipment thus constituting a sufficient danger that the immediate area should be cleared of all personnel for a time sufficient to recirculate the ambient atmosphere and remove the excess airborne lead particles therefrom.
To accomplish the measurement of the lead particles which may be present either in the air, the soil or other portions of the environment, it is recognized that x-ray fluorescence (xrf) is a very useful elemental analysis technique. Xrf uses common nuclear instrumentation to provide extra-nuclear indications of elemental composition both qualitatively and quantitatively. Xrf works best with medium high to high atomic number elements which includes lead.
The method usually uses gamma ray bombardment of the lead particles to excite x-ray emission from them and then uses scintillation or semiconductor detectors and computer-based multi-channel analyzers (MCA) to complete the elemental analysis. Small isotopic sources of gamma rays generally are preferred to x-ray sources of excitation.
One of the problems associated with xrf utilizing gamma ray excitation is that comparatively few of the gamma rays will be observed by traces of lead in the field of view of the xrf instrument head. A substantially larger fraction of the gamma rays emitted by the excitation source will be scattered and ultimately detected by the MCA as lower energy gamma rays.
The scatter gamma rays create a background noise and as a result will interfere with the desired x-ray signal. This results because gamma rays and x-rays are substantially the same in all ways except for their motive creation and the x-rays of value and the scattered gamma rays are combined in their distribution throughout the spectrum. Their combined presence in the spectrum presents data processing and/or instrument hardware requirements. One resolution of the difficulty of this combined scattered gamma ray and desired x-ray information is to use semi-conductor detectors which greatly reduce the interference relative to that of scintillation detectors. However, to use the semi-conductor detectors substantially increases the cost of the instrument. As a result, scintillation detection systems are highly desirable in an instrument of the type with which the present invention is concerned.
It is thus desirable that an instrument utilizing a scintillation detection system in which appropriate controls are effected to substantially mitigate the interference of scattered gamma rays emitted by the excitation source with the x-ray signals received from the excited particles of lead which are to be detected is used. Without effective control the gamma rays will enter the ambient in all directions from the isotopic source and even if some containment mechanism is utilized, these gamma rays will reflect one or more times and still interfere with the operation of the instrument by providing improper activation of the scintillation detector.